When a battery of a battery operated device such as an electric shaver, portable radio or TV, desk cleaner or the like has run down unexpectedly, the user of such device will be disadvantaged. This is especially true if the battery has run down unexpectedly under circumstances where the user does not have a spare battery or charging equipment, or when the user is shaving just before going out.
In order to obviate the situation such as described above, various proposals have heretofore been presented. In the Japanese Patent Early Publication No. 59-136031 (1984), for example, there is disclosed an apparatus and a method for optically and/or acoustically indicating the charged state of a battery or a storage battery. The apparatus is capable of indicating under load free conditions, that the battery or the storage battery has discharged below a predetermined value. The predetermined value is set by taking into account that the battery or the storage battery is to be still at a level for supplying power to a load, for a nominal period of time. The additional provision for an inspection key or a manual switch to select a test position is unnecessary when the load current necessary to operate the apparatus is small.
Also, in the Japanese Patent Early Publication No. 59-106828 (1984), there is disclosed a signal indicator apparatus for indicating a specific charged state of a storage battery, wherein a memory circuit is provided to maintain a charge state indicator energized for a specific interval of time after a device using the storage battery as a power supply is switched off.
Further, in the Japanese Patent Early Publication No. 60-3875 (1985), there is disclosed a charging type, small-sized electric device comprising a power supply including first and second storage batteries, a load supplied power from the power supply, a change over switch for selectively connecting a first storage battery or a second one to the load, a charger for charging the power supply, and a plug for connecting the charger to a commercial AC power supply, wherein upon the application of power to the charger, a changeover switch is operated to select the first storage battery for charging.
Furthermore, in the Japanese Utility Model Early Publication No. 59-34447 (1984), there is disclosed a "RECORDER-REPRODUCER APPARATUS" comprising a casing containing a plurality of batteries, a connecting means for selectively connecting one of the batteries to a driving circuit, a detecting circuit for detecting that the voltage of the battery under use is lowered below a predetermined voltage level, an indicator for producing an alarm in response to an output signal from the detecting circuit, for indicating that the using battery should be changed, and a switching means for switching the connecting means so as to connect another battery to the driving circuit.
In apparatus in which a discharge state of a battery is alarmed or indicated by a light emitting diode or the like, however, the alarm or the indication tends to be overlooked. Such apparatus requires additional indicator or alarm means, and additional electric power to ensure operation thereof. On the other hand, in an apparatus in which a plurality of spare batteries are provided, a rather larger casing space for the batteries is required, resulting in increased costs.